The proposed research will extend published peer-reviewed preliminary findings of the Principal Investigator which suggest the importance of 1 MHz, focused, finite amplitude ultrasonic propagation in biological tissue. The transient thermoelectric technique will be employed for the measurement of the absorption coefficient in tissue in vitro as well as in vivo at 37 C as a function of irradiating intensity over the 10 to 1000 Wcm-2 spatial peak-temporal peak intensity range. Simultaneous measurement of the spectral content of the irradiating field via Fast Fourier analyses of the digitized output of a calibrated wide-band hydrophone will enable the examination of the role of harmonic generation in the previously observed (by the Principal Investigator) increase in tissue ultrasonic absorption coefficient with intensity. Experiments are proposed to separate finite amplitude effects originating in the irradiating field and from within the tissue. The health-relatedness of the proposed research stems from a number of areas. While ultrasound is currently in widespread use in the fields of medical diagnosis and therapy, many questions regarding the manner in which ultrasound is propagated in biological tissue remain unanswered. Recent intensity measurements have determined that many medical diagnostic ultrasound instruments utilize focal fields at spatial peak-temporal peak intensities in the 100 to 1000 Wcm-2 range. The presence of intensity-dependent ultrasonic absorption may alter currently held views on ultrasound exposure risk assessment. The dependence of ultrasonic absorption on sound intensity observed in the present preliminary study in a number of tissues suggests such propagation phenomena may offer a new ultrasonic tissue characterization parameter which could offer an innovative noninvasive technique for examining disease states. The manner in which finite amplitude ultrasound propagates within tissue, and the degree to which the physiology of living systems is altered by such exposure, is currently of interest. This study will provide a significant contribution to the understanding of ultrasonic propagation in biological tissue and the importance of finite amplitude acoustics in medical ultrasound.